A CRT display used to be in the mainstream of conventional display devices, however, in place of the CRT display, a flat display, such as a liquid crystal display, plasma display, and so forth, has since been put to commercial use, and the demand for the flat display has been on the increase. Further, there have been advances in development and commercial use of a display utilizing organic electroluminescence {hereinafter referred to also as an organic EL display (OLED)}, and a display for forming images by disposing electron sources utilizing field emission in a matrix fashion to thereby shine florescent substances disposed at anodes (FED display).
The organic EL display has features in that since it is of a self-emission type in contrast to the liquid crystal display, a backlight is unnecessary (1), since a voltage necessary for emitting light is as low as 10 V, or less, there is a possibility of reducing power consumption (2), since a vacuum structure is unnecessary in contrast to the plasma display, and the FED display, the organic EL display is suited for reduction in weight, and lower profile (3), since response time is as short as several microseconds, the organic EL display is excellent in moving-picture characteristics (4), a viewing angle is as wide as 170 degrees, or wider (5), and so forth.
Thus, the organic EL display has the features described as above; however, one of problems with the organic EL display is a phenomenon that light-emission characteristics of organic EL elements (hereinafter referred to as OLED elements) vary along with the elapse of operation time. Further, there are cases where when a specific image is displayed for long time, variation in the characteristics of the OLED element appears as deterioration in the characteristics of part of the image only, so-called “image persistence”. The phenomenon of the image persistence is quite conspicuous in comparison with the case of gradual decrease in luminance intensity of a screen as a whole. In order to prevent the image persistence from becoming conspicuous, it is necessary to detect the characteristics of the OLED elements for all images to thereby feedback results of detection to an input signal delivered from a host.
Variation in the characteristics of the OLED element shows itself as variation in voltage-current characteristics of the OLED element. In other words, even if an identical voltage is applied to the OLED element, current flowing therethrough will decrease in amperage along with operation time. This holds true for not only the case where the operation time refers to a long time period, such as a service life, during which deterioration occurs to the characteristics of the OLED element, but also the case where the operation time refers to a relatively short time period such as the case of the image persistence. This phenomenon is shown in FIG. 20. In FIG. 20, the horizontal axis indicates voltage applied to the OLED element, ad the vertical axis indicates current flowing therethrough. In the figure, a curve A indicates initial characteristics of the OLED element, and a curve B indicates characteristics thereof after the elapse of time. Since light emission of the OLED element may be considered proportional to current flowing therethrough, luminance intensity of light emission of the OLED element undergoes variation along with the elapse of time even if an identical voltage is applied thereto, resulting in failure for displaying an accurate image.
Conversely, it follows that in order to cause an identical current to flow for causing identical light emission, it is necessary to apply a higher voltage. FIG. 21 shows variation in applied voltage, necessary for causing the same current to flow through the OLED element. In FIG. 21, the horizontal axis indicates operation time, and the vertical axis indicates applied voltage for causing a constant current to flow through the OLED element. FIG. 21 shows that the applied voltage should be increased in order to cause the identical current to flow through the OLED element.
As described above, with the organic EL display, in order to effect displaying of correct images, it is necessary to periodically measure the voltage-current characteristics of the OLED elements for all pixels to be thereby fed back to image signals as inputted. Reference literatures describing such technologies as described include JP-A No. 2005-156697, and JP-A No. 2002-341825.